This invention relates generally to optical devices, and more particularly to spatial light modulators.
Spatial light modulators (SLMs) are currently used in a variety of optical systems. They can be used for controlling (modulating) the intensity of an incident optical signal. SLMs operate as either as transmissive or reflective devices. A flat panel laptop computer screen is an example of a two-dimensional array of transmissive SLMs. Most SLMs have relatively slow switching speeds. For example, SLMs made with liquid crystals generally have speeds from a hundred hertz to a few kilohertz. The speed of liquid crystal SLMs are limited by the time required to align (transmissive) or twist (reflective) liquid crystals under the influence of an applied electrical field.
Acoustic-optic modulators (AOMs) use acoustical standing waves to modulate light and can have switching speeds of up to a few megahertz, but these devices are also bulky. Relatively new micro electromechanical structure (MEMS) micro-mirror SLM switches can operate in the kilohertz to megahertz range, but are usually bulky as well.
Small optical structures have been developed which rely interactions between a periodic medium which has a periodicity comparable to (or smaller than) the wavelength of electromagnetic waves traveling therethrough. In this case, the medium begins to significantly inhibit the wave""s propagation. Sub-wavelength grating structures (SWS) are a type of optical structure that rely on this sub-wavelength effect.
An example of the formation and several applications for a SWS is described in U.S. Pat. No. 6,035,089, to Grann, et. al. (xe2x80x9cGrannxe2x80x9d), which is assigned to Lockheed Energy Research Corporation, predecessor to the assignee of this application. The entire contents of U.S. Pat. No. 6,035,089 is hereby incorporated by reference. Grann describes a subwavelength resonant grating structure (SWS) formed from a single subwavelength resonant grating layer (SWL), which functions as a zeroth order diffraction grating. Grann""s SWS uses periodically spaced high refractive index xe2x80x9cpostsxe2x80x9d embedded in a lower refractive index dielectric waveguide material to form an extremely narrowband resonant reflector that occupies a very small area.
A SWS which functions as a zeroth order diffraction grating can be represented by an effectively uniform homogeneous material having an effective refractive index (neff). Under particular incident wave configurations, such as a substantially normal incident beam, and certain structural constraints, such as the refractive index of the medium surrounding the SWS less than refractive index of the waveguide less than refractive index of the posts, a SWS may exhibit a resonance anomaly which results in a strong reflected beam which is limited to an extremely narrow bandwidth. If the incident radiation is not within the SWS resonant bandwidth, most of the energy of the incident beam will propagate through the grating in the form of a transmitted beam.
This resonance phenomenon occurs when electromagnetic radiation is trapped within the grating material due to total internal reflection. If this trapped radiation is coupled into the resonant mode of the SWS grating, the field will resonate and redirect substantially all of the electromagnetic energy backwards. This resonance effect results in a nearly total reflection of the incident field from the surface, which may be designed to be extremely sensitive to wavelength.
If SLMs could be configured using SWS, improved SLM performance could result. This could benefit existing SLM applications and provide new applications for SLMs that were never before possible.
A reflective coherent spatial light modulator (RCSLM) includes a subwavelength resonant grating structure (SWS). The SWS is formed from at least one subwavelength resonant grating layer (SWL), the SWLs having a plurality of areas defining a plurality of pixels. Each pixel represents an area capable of individual control of its reflective response. The pixels are adapted to produce a resonant reflective response characterized by reflecting a band of incident light while transmitting light outside the reflective band. A structure for modulating the resonant reflective response of at least one pixel is also provided.
The structure for modulating can include at least one electro-optic layer, the electro-optic layer disposed in optical contact with at least one SWL. In this embodiment, the RCSLM can include a structure for modulating the refractive index of the electro-optic layer.
The RCSLM can include a first electrically conductive layer and a second electrically conductive layer, the electro-optic layer positioned substantially between the electrically conductive layers. At least one of the electrically conductive layers can include a plurality of plates, the plurality of plates substantially electrically isolated from one another and adapted to control the resonant reflective response of individual ones of the plurality of pixels.
The RCSLM can be formed on a die and can have electronic components provided thereon, including control electronics and a high speed voltage driver circuit, to permit the plurality of pixels to be separately switchable. Switching can be accomplished by application of a voltage signal generated by the high speed voltage driver circuit to the plurality of plates. The plurality of plates can be adapted for connection to the electronic components through back plane via connections. The RCSLM can include an anti-reflective coating layer disposed on the surface of the RCSLM.
In the embodiment having electrically conductive electrodes and at least one electro-optic layer therebetween, an anti-reflective coating layer can be disposed on the surface of the RCSLM and a buffer layer can be disposed between the anti-reflective coating layer and the second conductive layer. The buffer layer can be selected from air, SiO2, TiO2, and WO.
SWLs can include a plurality of features, at least a portion of the features having asymmetric exposed surfaces. In this embodiment, The RCSLM can include an electronically controllable quarter wave plate.
In the embodiment having electronic components formed on die, the electronic components can also include an amplitude controller for controlling the amplitude of light reflected from the plurality of pixels. The amplitude controller can include a feedback and control system. The feedback and control system can be adapted to separately control the amplitudes of light reflected by the pixels.
A method for forming a reflective coherent spatial light modulator (RCSLM), includes the steps of selecting a waveguide material having a first refractive index, and forming at least one subwavelength resonant grating structure (SWS), the SWS formed from at least one subwavelength resonant grating layer (SWL) in the waveguide material. The SWLs each have a plurality of areas defining a plurality of pixels, the plurality of pixels adapted to produce a resonant reflective response characterized by reflecting a band of incident light while transmitting light outside the reflecting band. The SWL can include a plurality of features formed from at least one material having a second refractive index greater than the first refractive index of the waveguide material.
The method can include the step of providing at least one electro-optic layer, the electro-optic layer disposed in optical contact with at least one SWL. The method can include a first electrically conductive layer and a second electrically conductive layer, the at least one electro-optic layer positioned substantially between the electrically conductive layers. At least one of the electrically conductive layers can include a plurality of electrically conductive plates, the plates substantially electrically isolated from one another and adapted to control the resonant reflective response of individual pixels.
The method can include the step of providing a bulk substrate material having a plurality of die, wherein a plurality of RCSLMs are formed on the plurality of die. The method can include the step of forming electronic components including control electronics and a high speed voltage driver circuit on the die, wherein the pixels can become separately switchable by application of a voltage signal from the high speed voltage driver circuit to conductive electrode plates to impose an electrical field across at least one electro-optic layer. The method can include the step of forming a plurality of back plane via connections, where the plurality of plates can be provided electrical connection to the electronic components. At least one SWL can include a plurality of features, at least a portion of the plurality of features having asymmetrical exposed surfaces.
The invention can be used in embodiments which use multiple RCSLMs. For example, a high rate projection image system includes at least three RCSLMs, each RCSLM adapted to reflect a different wavelength band. Each RCSLM includes a subwavelength resonant grating structure (SWS). The SWS is formed from at least one subwavelength resonant grating layer (SWL), the SWLs having a plurality of areas defining a plurality of pixels. The pixels are adapted to produce a resonant reflective response characterized by reflecting a band of incident light while transmitting light outside the reflective band. A structure for modulating the resonant reflective response of at least one pixel is also provided.
A structure for providing light beams supplies light to each RCSLM, the light beams providing wavelengths within the respective RCSLM wavelength bands. A viewing screen receives reflected light from the RCSLMs.
In another embodiment of the invention, a method for modulating light includes providing a reflective coherent spatial light modulator (RCSLM). The RCSLM includes a subwavelength resonant grating structure (SWS), the SWS formed from at least one subwavelength resonant grating layer (SWL), the SWLs having a plurality of areas defining a plurality of pixels. At least one pixel is adapted to produce a resonant reflective response characterized by substantially reflecting a band of incident light. A structure for modulating the resonant reflective response of the at least one pixel is provided. The reflective response of the at least one pixel is modulated, whereby the band of light is substantially transmitted by at least one pixel.
The modulation can be electronic modulation. Preferably, the electronic modulating is electro-optic modulation. Electro-optic modulation can be implemented by applying a voltage to at least one electro-optic layer, the electro-optic layer disposed in optical contact with at least one pixel. The voltage applied is preferably capable of variation, wherein the voltage variation can modulate the reflective response of at least one pixel.
The plurality of pixels can be separately controllable and adapted to produce a plurality of distinct resonant reflective responses. This permits the step of separately modulating the reflective responses of the plurality of pixels. Preferably, the modulation is electro-optic modulation.
A method for processing an electromagnetic signal comprises the step of utilizing a reflective coherent spatial light modulator (RCSLM) which includes a subwavelength resonant grating structure (SWS). The SWS is formed from at least one subwavelength resonant grating layer (SWL), the SWLs having a plurality of areas defining a plurality of pixels. Each pixel represents an area capable of individual control of its reflective response. The pixels are adapted to produce a resonant reflective response characterized by reflecting a band of incident light. A structure for modulating the resonant reflective response of at least one pixel is also provided. The utilizing step can include at least one selected from the group consisting of image projecting systems, 3-D holographic projecting and communications transceiving.
A coherent spatial light reflector adapted for fixed frequency operation includes a subwavelength resonant grating structure (SWS). The SWS is formed from at least one subwavelength resonant grating layer (SWL), the SWLs having a plurality of areas defining a plurality of pixels. The pixels are adapted to produce a resonant reflective response characterized by reflecting a band of incident light while transmitting light outside the reflective band.